1. Field of the Invention
The invention relates to an inkjet head which attains desirable images by jetting ink onto a recording medium, and more particularly, to an inkjet head for heating and jetting high viscosity ink.
2. Description of Related Art
An inkjet printer which jets ink onto a recording medium, e.g. a paper, a thin plastic plate or the like, to record a predetermined image has been proposed, and is now in practical use. The inkjet printer, which is equipped with an inkjet head having ink jetting openings, records a predetermined image on a recording medium by jetting ink onto the recording medium while the inkjet head is moved along a predetermined direction.
The inkjet printer jets ink through an inkjet head that utilizes a piezoelectric element. The inkjet head includes an inkjet head chip having ink jetting openings which are lined up to form a nozzle row on a front surface of the inkjet head chip, and a manifold attached securely to a side of the inkjet head chip and leading ink to the inkjet head chip.
However, because the ink used in the inkjet printer becomes high in viscosity when the temperature becomes low, it is required to keep the ink at a predetermined viscosity by heating the ink to decrease the viscosity so that the ink can be well jetted.
In recent years, a method for recording an image by an inkjet printer which uses UV curable ink that hardens when exposed to UV irradiation, has been proposed. But due to the fact that UV curable ink has a higher viscosity than general ink, it is difficult to jet highly viscous ink properly. Actually, an ink jetting method is under development, wherein UV curable ink is jetted, when heated into a low viscosity state, from an inkjet head.
For these reasons, it is well known that an ink heater disposed in an inkjet printer is used to heat ink for purposes of keeping ink at a certain temperature, transforming ink into liquid phase, and so on.
Known as examples of arrangement of ink heaters are: disposing an ink heater in an ink tank; disposing an ink heater for heating the filter case of the inkjet head as described in JP-Tokukaihei-10-790A; and so on.
In JP-Tokukai-2003-165217A, a technique is disclosed, wherein an elongated rod-shaped heater, which is longer than the longitude of a nozzle row, is embedded in a common ink chamber in parallel to the nozzle row with adhesives.
In addition, in JP-Tokukai-2003-136756A, another technique is disclosed, wherein a U-shaped plane heater is embedded in the wall of a common ink chamber while a circular cylindrical heater is disposed inside a common ink chamber.
In JP-Tokukaihei-7-276635A, a third technique is disclosed, wherein a layer having a high thermal conductivity is disposed between an ink heater and an inkjet head so that the ink can be heated equally, even if the ink heater is placed at a position that is inequidistant from each nozzle.
However, in the above mentioned ink heater which is disposed in an ink tank, the ink may change in temperature before being jetted from an ink jetting opening, and therefore the ink may be not well jetted.
In the case of the ink heater for heating the filter case of an inkjet head as described in JP-Tokukaihei-10-790A, it is difficult to vary the amount of heat corresponding to ink temperatures, and this makes it troublesome to maintain the ink at a predetermined temperature. Therefore a poor ink jetting may be caused.
One approach to solve the aforementioned problems is to maintain the ink at a predetermined temperature by heating the ink in the manifold through an ink heater which is disposed on a side portion of the manifold.
However, the ink in the manifold has an area where the temperature tends to be difficult to decrease and an area where the temperature tends to be easy to decrease because the heat dissipating condition in the inkjet head varies from place to place. For example, the temperature at the area around both end sides of the nozzle row tends to be easy to decrease, and the temperature in the area close to the ink inlet opening that flows the ink into the manifold also tends to be easy to decrease.
However, because the ink heater disposed on the side portion of the manifold heats all parts of the manifold in the same manner and the heat generation density is the same regardless of whichever position in the heater, it causes a variation to occur in temperature of the ink in the inkjet head. As a result, a variation in ink viscosity occurs, and therefore the ink droplets may be jetted unequally and a poor image formation may be caused.
A rod-shaped heater embedded with silicon adhesives is described in JP-Tokukai-2003-165217A. In this technique, while silicon resin has a relatively good thermal conductivity, it has difficulty in transferring heat equally from the heater to the ink because the thermal resistance of the adhesives is large, and therefore the heat transfer efficiency is poor. In addition, unlike a plane film type heater, a rod-shaped heater cannot heat the ink equally, because it has a small heat generation surface.
In JP-Tokukai-2003-136756A, a technique in which a plane heater is embedded in the wall of a manifold is disclosed. No concrete manner to embed the heater therein, however, is disclosed. After all, it is difficult to extend a thin plane heater to be even so as to embed it in a surface because it curls up.